In a nuclear reactor, for example a boiling water reactor, nuclear fuel rods are grouped together in an open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. In each assembly or bundle, fuel rods are supported between upper and lower tie plates in side-by-side parallel arrays. Spacers are employed at predetermined elevations along each fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation, and to protect the fuel rod assembly during possible loading events, such as handling and shipping.
Typical spacers often include a plurality of ferrules arranged in side-by-side relation and secured by, for example, welding to one another to form the support matrix of the spacer. A single fuel rod passes through each generally cylindrically shaped ferrule. The ferrules include circumferentially spaced, axially extending interior protuberances (or hard stops) and spring assemblies seated in openings formed in opposite sides of the ferrule from the protuberances, for centering and biasing the fuel rods against the hard stops, thereby maintaining the fuel rods in fixed relation one to the other across the spacer. The spacer itself constitutes an obstacle to bundle performance in that its cross-section interferes with the flow of water/moderator through the bundle. An ideal spacer would have minimal impact on bundle performance (thermal hydraulics, critical power), while still restraining the rods in their intended positions and protecting them. Consequently, an optimum fuel bundle spacer should have as little cross-section as possible, use a minimum amount of material and simultaneously meet structural requirements for positioning and protecting the fuel rods.
In developing new spacer spring designs for denser bundle matrices (for example, 8.times.8, 9.times.9, and 10.times.10), one challenge is to design the spring so that it will be sufficiently flexible to maintain historical preload limits as the space between the fuel rods becomes smaller (i.e., the spring deflection increases as the space decreases). Since it has been determined that the accelerated dead weight of the fuel rods at the. spacer locations damages the springs, a second challenge is to design the spring such that assembled fuel bundles can be shipped without the aid of plastic inserts to carry the weight of the rods as they travel on trucks. The damage mentioned above occurs because current spacer designs involve one spring being shared between two adjacent fuel rods, and this type of arrangement means that some fuel rods are sitting on the springs when the bundle is laid horizontally on the truck bed, causing the rod's own dead weight to be accelerated directly into the spring underneath it. Some prior spacer designs which include ferrules with one spring per two adjacent fuel rods are disclosed in commonly owned U.S. Pat. Nos. 5,173,252 and 5,078,961.
In commonly owned application Ser. Nos. 08/380,591 filed Jan. 30, 1995 and 08/516,203 filed Aug. 17, 1995, spring designs are disclosed which are based on a one spring per fuel rod criteria, with the ability to ship without conventional plastic inserts. However, a disadvantage of these spring designs is that too much spring material protrudes into the subchannel regions between the ferrules, and their respective geometries are thought to be susceptible to self-vibration due to coolant water flow across the springs.